Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
复制标题
DOI：
10.1029/2011jb008444
复制
发表时间：
2011-12
影响因子：
--
通讯作者：
Junshan Xu;D. Yamazaki;T. Katsura;Xiaoping Wu;P. Remmert;H. Yurimoto;S. Chakraborty
中科院分区：
文献类型：
--
作者：
Junshan Xu;D. Yamazaki;T. Katsura;Xiaoping Wu;P. Remmert;H. Yurimoto;S. Chakraborty
[1]在下地幔条件下，同时测量了镁硅钙钛矿单晶中硅和镁的自扩散系数。用单晶直接测量的硅体积扩散率(本研究)与用多晶实验(早期研究)测得的硅体积扩散率几乎没有区别。这项研究之间的一致证明了在钙钛矿中测量的硅扩散系数的可靠性。在我们测量的不确定范围内，硅或镁扩散的各向异性都不能被解决。钙钛矿中的扩散符合阿累尼乌斯方程，在25 GPa时，D=D_0exp(−ΔH/RT)，D_0=5.10×10~(-10)−~(11)m~2/S，D_0=4.99×10~(-10)−~(11)m~2/S，ΔH=308kJ/m ol，M_g=305kJ/m o l。镁在硅酸镁钙钛矿中的扩散系数明显低于橄榄石、水滑石和环木石中的扩散系数。我们发现，镁在钙钛矿中的扩散系数与硅非常相似。因此，下地幔的流变性可能受硅和镁的耦合运动控制。讨论了一个基于点缺陷的模型，该模型可以解释硅和镁在钙钛矿结构中的扩散行为。我们的数据表明，在温度、晶粒度和应力状态的实际范围内，在下地幔中既可以观察到扩散蠕变，也可以观察到位错蠕变。
[1] Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO3 perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D = D0 exp (−ΔH/RT) at 25 GPa, with D0 = 5.10 × 10−11 m2/s for Si and 4.99 × 10−11 m2/s for Mg, ΔH = 308 kJ/mol for Si, and 305 kJ/mol for Mg. Mg diffusivity in MgSiO3 perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect–based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO3 perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle.